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The global chemistry-climate model CAM-Chem has been extended to incorporatean expanded bromine and iodine chemistry scheme that includes naturaloceanic sources of very short-lived (VSL) halocarbons, gas-phasephotochemistry and heterogeneous reactions on aerosols. Ocean emissions offive VSL bromocarbons (CHBr3, CH2Br2, CH2BrCl,CHBrCl2, CHBr2Cl) and three VSL iodocarbons (CH2ICl,CH2IBr, CH2I2) have been parameterised by a biogenicchlorophyll-a (chl-a) dependent source in the tropical oceans (20° N–20° S).Constant oceanic fluxes with 2.5 coast-to-ocean emission ratiosare separately imposed on four different latitudinal bands in theextratropics (20°–50° and above 50° in both hemispheres).Top-down emission estimates of bromocarbons have been derived usingavailable measurements in the troposphere and lower stratosphere, whileiodocarbons have been constrained with observations in the marine boundarylayer (MBL). Emissions of CH3I are based on a previous inventory andthe longer lived CH3Br is set to a surface mixing ratio boundarycondition. The global oceanic emissions estimated for the most abundant VSLbromocarbons – 533 Gg yr−1 for CHBr3 and 67.3 Gg yr−1 forCH2Br2 – are within the range of previous estimates. Overall thelatitudinal and vertical distributions of modelled bromocarbons are in goodagreement with observations. Nevertheless, we identify some issues such asthe reduced number of aircraft observations to validate models in theSouthern Hemisphere, the overestimation of CH2Br2 in the uppertroposphere – lower stratosphere and the underestimation of CH3I in thesame region. Despite the difficulties involved in the global modelling ofthe shortest lived iodocarbons (CH2ICl, CH2IBr, CH2I2),modelled results are in good agreement with published observations in theMBL. Finally, sensitivity simulations show that knowledge of the diurnalemission cycle for these species, in particular for CH2I2, is keyto assess their global source strength

Bromine and iodine chemistry in a global chemistry-climate model: description and evaluation of very short-lived oceanic sources